Superheterodyne receiving circuit



Patented May 14, 1940 UNITED STATES PATENT oricE" SUPERHETERODYNE RECEIVING CIRCUIT poration of Delaware Application December 16, 1938, Serial No. 246,086 In the Netherlands May 17, 1938 10 Glaims.

This invention relates to a superheterodyne receiving circuit arrangement of which themixing tube has an electrode-system, which serves for frequency-transformation of the incoming oscillations and comprises at least one input control-grid to which the received oscillations are supplied, one oscillator control-grid to which the local oscillations are supplied, and one ormore screen-grids placed between these two control rids.

It is generally known in low-frequency amplification to make use of negative back coupling for the avoidance of distortion due to curvature of the tube characteristics.

In the high-frequency part of radio receivers difficulties occur also due to curvature of the tube characteristics such as, for instance, the

so-called cross-modulation. It has already been proposed to meet these difiiculties by making use of negative back-coupling in the high-frequency amplifier. However, this method of 'doingaway with cross-modulation could not be used hitherto if cross-modulation occurs in the mixing tube. In fact, the use of the usual circuit-arrangements for negative back-coupling in the mixing stage of a radio-receiver generally involves undesired couplings between the receiving circuit and the oscillator circuit, which couplings maylead inter alia to a decrease in conversion slope and to radiation of the local oscillations by the antenna.

Even if no strong cross-modulation occurs in the mixing stage it is often desirableto use negativeback-coupling in the mixing stage and this to avoid the production of harmonics of the incoming frequencies which may give rise to the production of annoying inteference tones.

The object of the present invention is to provide a mixing circuit-arrangement for superheterodyne receivers in which negative back-coupling is used and in which undesired couplings between the oscillator-circuit and the receiving circuit-are avoided.

According to the invention use is made for this purpose of a circuit-arrangement in which the input control-grid approaches the. cathode more closely than does theoscillator control-grid and which includes a back-couplingimpedance which is traversed by the-alternating current of all current-carrying electrodes of they electrodesystem for frequency-transformation,.said impedance being included atthe same time in the circuit of the input control-grid or being coupled to this circuit so as to ensure negative back-coupling for at least a part of the :received oscillations, the local oscillations .being appliedtbetween the oscillator control-grid and that end of the back-coupling impedance which is connected to the cathode.

The invention is. based on the recognition that in multi-grid tubes having two control-grids that are separated from each other by one or more screen-grids, the control of the cathode current is substantially exclusively effected by the tension of the inner control-grid. If the inner control-grid is used as an input control-grid in a mixing circuit-arrangement the cathode current will consequently substantially exclusively depend on the incoming oscillations so that a voltage will be set up acrossa back-coupling impedance inserted in the cathode lead which voltage comprises only the frequencles of the incoming oscillations and not the local frequency. By connecting the two control-grid circuits in such a manner that the back-coupling voltage occurs in the circuit of the input control-grid, but not in the circuit of the oscillator control-grid, all undesired couplings can be avoided.

By combining the above measures it is consequently possible to make use of negative backcoupling also in the mixing stage of a superheterodyne receiving circuit-arrangement.

According to the invention a separate oscillator tube is preferably used, the oscillations set up in the oscillatory circuit of the oscillator being inductively transmitted to an inductance connected between the oscillator control-grid and that end of the back-coupling impedance which is connected to the cathode. This circuibarrangement has the particular advantage that the tuning condensers of the input circuitand of the oscillatory circuit of the oscillator can both be connected to each which in another method of coupling between the oscillatory circuit of the oscillator and the circuit of the oscillator control-grid would give rise to a short-circuit of the backcoupling impedance.

For generation of the local oscillations and frequency-transformation of the incoming oscillations a multiple discharge tube may also be used which comprises an oscillator-system and a mixing system having a cathode which is common to both systems. The triode-hexode is an exam- .ple of such a multiple tube. According to a further feature of the invention undesired couplings are avoided therein by connecting both the control-grid circuit and the anode circuit of the oscillator system for high frequencies to the end of the back-coupling impedance which is connected to the cathode.

Although in the usual tubes comprising two lead which is remote from the cathode.

control-grids separated from each other by one or more screen-grids the condition that control of the cathode current should be effected only by the voltage of the inner control-grid generally has already been satisfied to a sufficient degree of accuracy, it may be advisable in some cases so to construct the electrode-system for frequency transformation and/or to apply such biases to the different electrodes of this system that substaritially all of the electrons reversing their direction in the vicinity of the oscillator control-grid are intercepted by the screen-gr1d(s) placed between the two control-grids. By this measure it is avoided that the said electrons reversing their direction adjoin the space-charge set up in the vicinity of the input control-grid and thus affect the cathode current, which might cause the oathode current to be controlled by the local oscilla' tions. I

The invention will be more clearly understood by reference to the accompanying drawing representing, by way of example, two forms of construction thereof. Figure l is a circuit diagram of a superheterodyne receiver having a multigrid mixing tube, and Figure 2 is a circuit diagram of a superhetcrodyne receiver having a multiple discharge tube.

Fig. 1 shows a frequency-transformation circuit-arrangement which comprises a mixing tube I having a cathode 2, an input control-grid 3, a screen-grid 4, an oscillator control-grid 5, a second screen-grid 6, a suppressor-grid I, which is connected to the cathode, and an anode 8. The antenna 9 is connected to earth through the intermediary of an inductance II], which is inductively coupled to a second inductance II. The coil I I forms part of an oscillatory circuit, which by means of a variable condenser l2 can be tuned to the frequency of the oscillations to be received and is connected to the input control-grid 3. The anode-circuit of the tube I includes an intermediate-frequency circuit I3 which is coupled to a second intermediate-frequency circuit I4. The intermediate-frequency output-voltage is taken from the terminals I5 and I6.

The anodecircuits and the circuits of the two screen-grids 3 and 6 are connected for high-frequency through a condenser I! to that end of a feedback impedance I8 inserted in the cathode This feedback impedance I8 is included at the same time in the circuit of the input control-grid 3 and is traversed by the alternating current of all current-carrying electrodes of the tube I. Since the cathode current substantially exclusively depends on the voltage of the input control-grid 3,the voltage set up across the feedback impedance I8 comprises substantially exclusively the frequencies of the incoming oscillations and not the local frequencies.

In the drawing the feed-back impedance isrepresented by an ohmic resistance which is not short-circuited for high frequency. Instead of an ohmic resistance it is possible to use another feed-back impedance such as a strongly damped oscillatory circuit which is tuned to a frequency lying about in the middle of the range of frequencies to be received. When the negative backcouping serves only for avoiding the production of harmonics giving rise to annoying interference tones it is also possible to use a frequency-dependent negative back-coupling, for instance by using for the feed-back impedance an oscillatory circuit which is not too strongly damped and tuned to an interference-frequency causing interference-tones. One example of such an interference-frequency is the frequency which amounts to four times the intermediate frequency, which frequency upon reception of a signal whose frequency amounts to about double the intermediate frequency, often causes annoying whistling noises. Upon reception of a signal whose frequency amounts to about double the intermediate frequency the local oscillator, in-

deed, is tuned to a frequency amounting to about thrice the intermediate frequency. Hence, the

A second harmonic of the signal to be received together with the oscillator frequency may yield a difference frequency which only very slightly differs from the intermediate frequency and which consequently together with the produced intermediate frequency may cause an audible interference tone.

The production of this interference tonecan be prevented by using as a feedback impedance an oscillatory circuit which is tuned to the second harmonic of the double intermediate frequency, 1. e. to a. frequency amounting to four times the intermediate frequency.

With superheterodyne receivers whistling noises are furthermore produced upon reception of a signal whose frequency is about one half of the intermediate frequency. In this case the second harmonic of the signal to be received differs only slightly from the produced intermediate frequency and may consequently yield an audible difference tone relatively to the intermediate frequency. This interference tone can be suppressed by using for the feed-back impedance an oscillatory circuit tuned to the intermediate frequency.

If the feed-back impedance is an ohmic resistance such as is the case in the circuit arrangement shown in Fig. 1, it may principally serve also for the production of the required negative bias for the input control-grid 3. However, for the obtainment of sufficient negative back-coupling the back-coupling impedance must generally be of the order of magnitude of about 2000 to {i000 ohms which value is too high for the production of a suitable grid bias. To meet this drawback only part of the resistance I8 of the circuit-arrangement shown in Fig. l is interposed in the direct current circuit of the input controlgrid and this because an intermediate point of the resistance I8 is connected through a high resistance I 9 to the control-grid 5, whereas the end of the resistance I8 remote from the cathode is connected for high frequency to the control-grid 3- through a condenser 20 which forms part of the input oscillatory circuit. In this manner it is ensured that the total alternating voltage set up across the resistance I8, but only part of the dithe incoming oscillations use is made of a triodehexode 29 which comprises an oscillator system and a mixing system having a cathode 30 which is common to'both systems. The mixing system comprises aninput control-grid 3I, a screen-grid 32, an oscillator control-grid 33, a second screengrid 38, and an anode 35, whilst the oscillatorsystem comprises a control-grid 36 and an anode 31. A screen 43 may be placed between the two electrode-systems.

To obtain a negative back-coupling for at least part of the received frequencies the cathode lead comprises a back-coupling impedance 25 which is traversed by the alternating current of all ourrent-carrying electrodes of the mixing system. To this end the anode-circuit of the mixing system and the circuits of the screen-grids 32 and 34 are earthed for high frequency through condensers 21 and 28, whilst also the end of the feedback impedance 25 remote from the cathode is connected to earth through a condenser 39. Since the oscillatory-circuit ll, I2 is also connected to earth the feed-back impedance 25 is at the same time included in the circuit of the input control-grid 3i.

The feed-back impedance 25, which is constituted by a strongly damped oscillatory circuit, constitutes a short-circuit for direct current and consequently cannot serve for the production of a negative bias for the input control-grid. For this reason a resistance 26 is provided in series with the feed-back impedance, which resistance is short-circuited for high frequency and serves for the production of this bias.

The anode circuit of the oscillator system is connected for high frequency through a condenser 38 to the cathode 30 as Well as the control-grid circuit of the oscillator system. As a result of this it is ensured on the one hand that the alternating current of the oscillator-system does not traverse the feed-back impedance, whereas on the other hand the feed-back voltage is not set up in the control-grid circuit of the oscillatorsystem, thus avoiding all undesired couplings between both systems.

For suitable operation of the circuit arrangement referred to above it is advantageous that the current distribution effected by the oscillator control-grid 5 and 33 respectively depends as slightly as possible on the voltage set up at the input control-grid 3 and 3! respectively, which condition can be satisfied by a suitable construction of the electrode-system for frequency-transformation and/or by a. suitable choice of the biases applied to the electrodes of this system.

We claim:

1. Asuperheterodynereceivingcircuit-arrangement comprising a mixing tube provided with an electrode-system which serves for frequencytransformation of the incoming oscillations, and

which comprises at least one input control-grid, to which the incoming oscillations are supplied, one oscillator control-grid, to which the local oscillations are supplied, and one or more screengrids placed between these two control-grids, characterised in that the input control-grid is spaced closer to the cathode than the oscillator control-grid, a back-coupling impedance connected to the cathode and arranged to be traversed by the alternating current of all current carrying electrodes of the said electrode-system whereby a negative back-coupling is obtained for at least part of the incoming oscillations, the local oscillations being applied between the oscillator control-grid and that end of the back-coupling impedance which is connected to the cathode.

2. A superheterodyne receiving circuit-arrangement as claimed in claim 1, comprising a separate oscillator tube, in which the oscillations setup in the oscillatory circuit of the oscillator are inductively transmitted to an inductance connected between the oscillator control-grid and the end of the back-coupling impedance which is connected to the cathode.

3. A -superhet'erodyne receiving circuit-arrangement as claimed in claim 1, in which a multiple discharge tubeis used for generation of the local oscillations and frequency-transformation of the incoming oscillations, said discharge tube comprising an oscillator system, a mixing system and a cathode which is common to both systems, and in which both the control-grid circuit and the anode circuit of the oscillator system are connected for high-frequency to the end of the back-coupling impedance which is connected to the cathode.

4. In a frequency converter, the combination of a vacuum tube provided with a cathode and an anode, a plurality of control electrodes including a signal grid and an oscillator grid interposed between said cathode and anode, an input circuit connected between the signal grid and cathode, an output circuit connected between the anode and cathode, an impedance common to said input and output circuits connected to the cathode and adapted to produce negative feedback from said output circuit to said input circuit, a source of local oscillations, and means for impressing the local oscillations between the oscillator grid and that end of the feedback impedance which is connected to the cathode.

5. In a frequency converter, the combination defined in claim 4 wherein the impedance comprises an unbypassed resistance.

6. In a frequency converter, the combination defined in claim 4 wherein the impedance comprises an unbypassed resistance, and a portion of the voltage drop developed across said resistance is applied to the signal grid .as negative bias.

7. In a frequency converter, the combination defined in claim 4 wherein the impedance comprises a strongly damped oscillatory circuit.

8. In a frequency converter, the combination of a vacuum tube provided with a cathode and an anode, a signal grid, a screen grid and an oscillator grid interposed between said cathode and anode in the order named, an input circuit connected between the signal grid and cathode, an output circuit connected between the anode and cathode, an impedance common to said input, output and screen grid circuits connected to the cathode and adapted to produce negative feedback from said output and screen grid circuits to said input circuit, a source of local oscillations, and means for impressing the local oscillations between the oscillator grid and that end of the feedback impedance which is connected to the cathode.

9. In a frequency converter, the combination of a vacuum tube provided with a pentode section and a triode section which have a common cathode, the pentode section including a signal grid, an oscillator grid, a screen surrounding the latter grid, and an anode, the triode section including an oscillator grid connected to the corresponding grid of the pentode section, and an anode, an input circuit connected between the signal grid and cathode, an output circuit connected between the pentode section anode and cathode, an impedance common to said input and output circuits connected to the cathode and adapted to produce negative feedback from said output circuit to said input circuit, circuit elements connected to the grid and anode of the -triode section for producing'local oscillations, and means for impressing the localoscillations between the oscillator grid of the pentode section and that end of the feedback impedance which is connected to the cathode.

10.. In a frequency converter, the combination defined in claim 9 wherein the negative feedback impedance comprises a strongly damped oscillatory circuit.

JOHAN HAANTJES. BERNARDUS DOMINICUS HUBER'I'US TELLEGEN. 

